A lidar system with enhanced performance

ABSTRACT

A LIDAR system includes a transmitter, a receiver, and a processor. The transmitter directs a sequence of illumination pulses toward a scene. The receiver including an array of photodetectors, which receive optical radiation reflected from the scene and output respective signals in response to the received optical radiation, a shutter which modulates the signals output by the photodetectors by applying a chirp shutter function, having a selected chirp period, to the signals, and a readout circuit, which samples and digitizes the modulated signals in each of a plurality of sampling windows, which span the chirp period, thereby generating a corresponding plurality of digitized output signals. The processor selects respective sampling windows for the photodetectors, and processes the digitized output signals in the selected respective sampling windows to generate a depth map of the scene.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication 63/345,921, filed May 26, 2022, whose disclosure isincorporated herein by reference.

TECHNICAL FIELD

Embodiments described herein relate generally to LIDAR systems, andparticularly to methods and systems for enhancing performance of LIDARsystems.

BACKGROUND

Light Detection and Ranging (LIDAR) techniques are typically used fordetermining ranges to objects in a scene and for creating aThree-Dimensional (3D) model of the scene.

LIDAR systems are known in the art. For example, U.S. patent applicationSer. No. 16/949,835, whose disclosure is incorporated herein byreference, describes a chirp-based illumination LIDAR system having atransmitter that may include a pulsed radiation illuminator that isfollowed by a beam forming optics. The transmitter may be configured tooutput, during each illumination period of a sub-group of illuminationperiods, a first plurality of radiation pulses that form a decimatedchirp sequence of radiation pulses; the decimated chirp sequence is asparse representation of a chirp signal. A receiver of the system may beconfigured to receive, during each reception period of a sub-group ofreception periods, one or more received light pulses from one or moreobjects that were illuminated by the one or more radiation pulsestransmitted during each illumination period. The receiver may includemultiple radiations sensing elements, multiple shutter circuits, andmultiple processing circuits for converting the one or more receivedlight pulses to output information; wherein the multiple shuttercircuits may be configured to apply a shutter function on intermediatesignals, the intermediate signals represent radiation sensed by themultiple radiations sensing elements, wherein the shutter functionrepresents the chirp signal.

SUMMARY

An embodiment that is described herein provides a LIDAR system,including a transmitter, a receiver and a processor. The transmitter isconfigured to direct a sequence of illumination pulses toward a scene.The receiver including an array of photodetectors, a shutter, and areadout circuit. The photodetectors are configured to receive opticalradiation reflected from the scene and to output respective signals inresponse to the received optical radiation. The shutter is configured tomodulate the signals output by the photodetectors by applying a chirpshutter function, having a selected chirp period, to the signals. Thereadout circuit is configured to sample and digitize the modulatedsignals in each of a plurality of sampling windows, which span the chirpperiod, thereby generating a corresponding plurality of digitized outputsignals. The processor is configured to select respective samplingwindows for the photodetectors, and to process the digitized outputsignals in the selected respective sampling windows to generate a depthmap of the scene.

In some embodiments, the readout circuit is configured to integrate thesampled signals over each of the sampling windows. In other embodiments,the processor is configured to calculate a time-frequency transform forthe sampling windows in a given chirp period, and to select a samplingwindow in the given chirp period based on partial subsets of thefrequency-domain bins, corresponding respectively to the samplingwindows. In yet other embodiments, the processor is configured tocalculate the time-frequency transform by calculating only the partialsubsets of the frequency-domain bins.

There is additionally provided, in accordance with an embodiment that isdescribed herein, a LIDAR system, including a transmitter, a receiver,and a processor. The transmitter is configured to direct a sequence ofillumination pulses toward a scene. The receiver including an array ofphotodetectors, a shutter, and a readout circuit. The photodetectors areconfigured to receive optical radiation reflected from the scene and tooutput respective signals in response to the received optical radiation.The shutter is configured to modulate the signals by applying anascending chirp shutter function, having a selected chirp period, to thesignals output by at least a first group of the photodetectors andsimultaneously applying a descending chirp shutter function, having thesame chirp period as the ascending chirp shutter function, to a load.The readout circuit is configured to sample and digitize the modulatedsignals to generate digitized output signals. The processor isconfigured to process the digitized output signals to generate a depthmap of the scene.

In some embodiments the load includes a second group of thephotodetectors, different from the first group of the photodetectors.

There is additionally provided, in accordance with an embodiment that isdescribed herein, a LIDAR system, including a transmitter, a receiver,and a processor. The transmitter is configured to direct a sequence ofillumination pulses toward a scene. The receiver including an array ofphotodetectors, a shutter, and a readout circuit. The photodetectors areconfigured to receive optical radiation reflected from the scene and tooutput respective signals in response to the received optical radiation.The shutter is configured to modulate the signals output by thephotodetectors by applying a chirp shutter function, having a selectedchirp period, to the signals, the shutter includes a switching circuitcomprising a switched circuit comprising a differential track-and-holdor a differential sample-and-hold circuit, which is controlled to applythe chirp shutter function. The readout circuit is configured to sampleand digitize the modulated signals to generate digitized output signals.The processor is configured to process the digitized output signals togenerate a depth map of the scene.

In some embodiments, the switched circuit is configured to reduceflicker noise using a chopping technique. In other embodiments, theswitched circuit comprises an integrating sampler comprising atransconductor stage followed by a sampler.

These and other embodiments will be more fully understood from thefollowing detailed description of the embodiments thereof, takentogether with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram that schematically illustrates a LightDetection and Ranging (LIDAR) system, in accordance with an embodimentthat is described herein;

FIG. 2 is a block diagram that schematically illustrates data flow in achirp LIDAR system, in accordance with an embodiment that is describedherein;

FIG. 3 is a block diagram that schematically illustrates a receivermitigating ambient light by dividing chirp sections into multiplewindows, in accordance with an embodiment that is described herein;

FIG. 4 is a block diagram that schematically illustrates a variantimager, in accordance with an embodiment that is described herein;

FIG. 5 is a diagram that schematically illustrates a chirp sectiondivided into multiple windows, in accordance with an embodiment that isdescribed herein;

FIG. 6 is a diagram that schematically illustrates chirp signals andrelated average power consumption, in accordance with an embodiment thatis described herein;

FIGS. 7A and 7B are block diagrams that schematically illustrate imagersin which the (de)-modulator is implemented using a Track-and-Hold (TAH)circuit;

FIGS. 8A and 8B are block diagrams that schematically illustrate imagersin which the (de)-modulator is implemented using a Sample-and-Hold (SAH)circuit;

FIGS. 9A-9C are a block diagram and electronic circuit diagrams thatschematically illustrate an imager employing a differential TAH circuit,and example circuits implementing the differential TAH circuit, inaccordance with embodiments that are described herein; and

FIGS. 10A-10C are a block diagram and electronic circuit diagrams thatschematically illustrate an imager employing a differential SAH circuit,and example circuits implementing the differential SAH circuit, inaccordance with embodiments that are described herein.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

Light Detection and Ranging (LIDAR) systems are typically used forcreating a 3D model of a scene. In some types of LIDAR systems, thescene is illuminated using an infrared pulsed laser, and the distancesto objects in the scene are measured based on the time elapsing betweentransmitting the laser pulses and receiving corresponding reflectedpulses from the objects. LIDAR systems are applicable, for example, inself-driving vehicles and other automotive applications.

Embodiments that are described herein provide improved methods andcircuits in LIDAR systems.

In some embodiments, the transmitter applies to the pulses to betransmitted a chirp modulation signal that scans a predefined frequencyrange in the electrical domain (without modifying the laser wavelength).At the receiver, (de)-modulation of the received signal is carried outusing a chirp shutter signal that is synchronized to the chirpmodulation signal of the transmitter. The receiver evaluates distancesto the reflecting objects based on the beat frequency of the receivedsignal relative to the chirp shutter signal.

In some scenes, light emitted by the transmitter (illuminator) may reachthe receiver (imager) on several trajectories, causing multi-pathreflections. Since by using chirp modulation, multi-path reflections areassociated with different beat frequencies, the receiver can separateamong them.

Some scenes contain ambient light such as sunlight, which maysignificantly interfere with reception of the reflected light. One wayto mitigate ambient light could be to use powerful illuminators. Thisapproach, however, typically is inapplicable due to regulations relatedto eye safety. Other example methods to mitigate ambient light could (1)include a spectral filter at the receiver that passes only wavelengthsin the vicinity of the wavelength used by the underlying laser, (2)apply spatially non-uniform illumination to concentrate the transmittedlight only on a portion of the scene, (3) apply spatial non-uniformityof the receiver (imager) sensitivity, wherein readout is carried outonly for the illuminated portion of the scene, thereby avoiding ambientlight received when other parts of the field are illuminated, and (4)use a differential receptor with a shutter chirp having a duty cycle of50%. The signal is seen as the difference between the positive andnegative samples, which for the ambient light (except from the noisecomponent) are equal, so that the ambient light is cancelled out (beingorthogonal to the signal expected by the detector). Other examples mayuse coded pulses such as Walsh codes, Golay pairs or biorthogonal codes.

In some disclosed embodiments, the chirp signal that scans the entirefrequency range is divided into multiple chirp sections, each of whichis divided into multiple windows. The chirp sections scan correspondingsubranges of the total frequency range. In these embodiments, onlywindows containing a reflected signal are selected, thereby reducing theamount of noise related to ambient light. In an embodiment, the signal(de)-modulated by the chirp shutter signal is integrated separately foreach window across multiple chirp sections of a readout cycle. Thisintegration scheme improves Signal to Noise Ratio (SNR) and distancemeasurement accuracy.

In some embodiments, since the location of the windows within the chirpsection is related to depth, the processor is configured to calculate atime-frequency transform for the sampling windows in a given chirpsection, and to select a sampling window in the given chirp sectionbased on partial subsets of the frequency-domain bins, correspondingrespectively to the sampling windows. In one embodiment, e.g., to reducepower consumption, the processor is configured to calculate thetime-frequency transform by calculating only the partial subsets of thefrequency-domain bins.

The chirp shutter signal is typically distributed to multiple(de)-modulators in the imager. When the chirp shutter signal being usedis an ascending chirp signal that increases the frequency over time, thepower consumption increases with time as well. This effect isundesirable and may degrade image uniformity of the LIDAR system. Insome embodiments, to mitigate the effect of increasing the powerconsumption, the imager produces and applies a descending chirp shuttersignal in synchronization with the ascending chirp shutter signal. Thedescending chirp shutter signal is applied to a load similar to that ofthe ascending chirp shutter signal. For example, the ascending anddescending chirp shutter signals may be applied to a similar number of(de)-modulators in the imager. When applying both the ascending anddescending chirp shutter signals simultaneously, the average powerconsumption becomes essentially non-varying in time.

In some embodiments, the imager comprises a (de)-modulator perphotodetector. The (de)-modulator typically comprises an analog mixerthat multiplies the signal produced by the photodiode by the shuttersignal. In some embodiments, the mixer is replaced with a suitableswitched circuit such as a Track and Hold (TAH) circuit or a Sample andHold (SAH) circuit. Differential TAH circuits and/or differential SAHcircuits may also be used. The differential TAH and SAH are advantageousbecause they exploit the whole signal energy and eliminate flickernoise. In some embodiments, the switched circuit reduces flicker noiseusing a chopping technique. In other embodiments, the switched circuitcomprises an integrating sampler comprising a transconductor stagefollowed by a sampler.

In the disclosed techniques various approaches are employed forimproving the performance of a chirp-based LIDAR system. In oneapproach, to mitigate noise related to ambient light, chirp sections aredivided into multiple windows, of which only windows containingsignificant signal are selected, thereby improving SNR and distancemeasurements accuracy. In another approach a descending chirp shuttersignal is applied simultaneously with the ascending chirp shuttersignal, resulting in constant average power consumption. In yet anotherapproach, the receiver (de)-modulator is implemented using a switchedcircuit rather than an analog mixer. The disclosed embodiments may beapplied separately of in combination with one another.

System Description

FIG. 1 is a block diagram that schematically illustrates a LightDetection and Ranging (LIDAR) system 10, in accordance with anembodiment that is described herein.

LIDAR system 10 comprises a controller 14, a signal processor 20, aninterface 16, a transmitter 30 and a receiver 40. Controller 14 managesand coordinates the operation of the LIDAR system. Signal processor 20computes a 3D scene model of the underlying scene, based on reflectionsdetected by receiver 40, and outputs the 3D scene model via interface 16to an external processor, e.g., a main processor in a vehicle. Signalprocessor 20 may output the 3D scene model in any suitable format. Forexample, the 3D scene model is output in the form of a 3D point cloud,which comprises a discrete group of data points in a 3D space (e.g., aCartesian space).

In the example of FIG. 1 , controller 14 and signal processor 20 areimplemented as separate processing modules. In alternative embodiments,however, the functionalities of controller 14 and signal processor 20may be executed by a common processor.

Transmitter 30 is configured to illuminate a scene in front of the LIDARsystem with a light pattern. The transmitter comprises a modulated laserilluminator 32 and beam-forming optics 34. Laser illuminator 32 produceslight pulses in a desired light pattern, which is further shaped bybeam-forming optics 34. In some embodiments, the beam forming opticsgenerates a desired spatial distribution of the illuminant power.

Receiver 40 comprises an imager system 42 (also referred to herein justas an “imager” for brevity) and imaging optics 44. The imager systemcomprises an imaging pixel array 46, on which imaging optics 44 projectslight received from the scene. In some embodiments, pixel array 46comprises a specialized CMOS array. In the present context, the term“specialized” means that the pixels are (i) optimized for the wavelengthof the laser and (ii) support the bandwidth required to operate with themodulated light. Such pixels differ from conventional pixels that justintegrate the light received during the exposure time (thereby requiringa low bandwidth). For creating a 3D model of the scene, the imagersystem derives angular information and depth (e.g., distance)information from reflected pulses received. Based on the location of apixel in pixel array 46, the imaging system may determine angularposition to the relevant object.

In the present context, the term “pixel” refers to a light-sensitivesensor such as a photodetector, possibly together with an electroniccircuit such as a (de)-modulator, a filter, and a sampler.

The LIDAR system transmits laser pulses 70 and receives some of thetransmitted pulses that were reflected from objects in the scene. Theimager system acquires depth information by measuring the delay betweentransmission times of laser pulses 70 and corresponding reception timesof received laser pulses such as 71, 72 and 73.

LIDAR system 10 in FIG. 1 further comprises a timing engine 25 thatcontrols the operation of transmitter 30 and receiver 40. For example,the timing engine coordinates the transmission of laser pulses bytransmitter 30, and receiver aperture slots in imager 42.

In some embodiments, timing engine 25 provides chirp triggering signalsto illuminator 32 of the transmitter for controlling the generation ofchirp sequences of radiation pulses. Timing engine 25 further provideschirp shutter triggering signals to imager 42 for controlling aperturetime slots in the receiver.

LIDAR system 10 may exchange any suitable information with other systemsand/or devices, via interface 16. For example, an external device orsystem may exchange commands and notifications with controller 14, andconfiguration and data with signal processor 20.

In the example of FIG. 1 , the underlying scene has three objects 60,illuminated by the light pattern produced by the transmitter. Otherscenes may contain no reflecting objects, a single object, or any othersuitable number of objects other than three. In some scenarios, objects60 may be illuminated by additional light sources other than the LIDARsystem. For example, objects 60 may be illuminated by ambient(background) light, e.g., sunlight. The ambient light may be at leastpartly reflected by the objects in the scene and received in thereceiver along with the reflected laser pulses, Ambient light typicallyinterferes with reception of the reflected laser pulses.

In some embodiments, to reduce performance degradation due to ambientlight, the LIDAR system may transmit light pulses at a wavelength of (orclose to) 940 nm (infrared light). This wavelength is advantageousbecause it is highly absorbed by gases in the atmosphere, therebyresulting in reduced interference caused by sunlight. In someembodiments, the receiver may filter received light using a suitablebandpass filter (in the light domain), which may be part of imagingoptics 44. For example, such a bandpass filter may be centered about thewavelength of 940 nm and have a passband width of 20 nm. For Si-baseddetectors, a 940 mm wavelength is typically the most suitable. Whenother materials are used in the photodetector, (e.g., Ge, GaAs or InP),other dips in the atmospheric light spectrum might be exploited such as1100 nm, 1400 nm, 1900 nm, and the laser wavelength is selectedaccordingly. The bandpass filter may attenuate light outside thepassband by 40 dB, in an embodiment. Alternatively, other centerwavelengths, bandwidths, and attenuation factors for the receiverbandpass filter can also be used.

Embodiments for mitigating noise related to ambient light, which areimplemented using windowing in the digital domain, and otherimprovements applicable in LIDAR system 10, will be described in detailbelow.

Signal Flow in a Chirp Lidar System

A LIDAR system such as LIDAR system 10 of FIG. 1 measures the distanceto an object in a scene, by transmitting a light signal (e.g., laserpulses) and evaluating the time it takes the light signal reflected bythe object to return to the receiver. The time to the object and back isreferred to a Time of Flight (ToF). For an object located at a distance‘x’ from the LIDAR system, the ToF is given by ToF=2x/c, wherein ‘c’denotes the speed of light.

In chirp LIDAR system 10, the frequency of the transmitted signalchanges over time, e.g., in a linear manner. Consequently, theinstantaneous frequency of the received signal is indicative of thedistance to the object from which the transmitted signal was reflected.

FIG. 2 is a block diagram that schematically illustrates data flow in achirp LIDAR system 90, in accordance with an embodiment that isdescribed herein. The chirp LIDAR system in FIG. 2 , may be used inimplementing LIDAR system 10 of FIG. 1 . Several modificationsapplicable to LIDAR system 90 for improving performance will bedescribed in detail below.

Chirp LIDAR system 90 comprises transmitter 30 and receiver 40 (of FIG.1 ). Transmitter 30 transmits a light signal denoted P(t) forilluminating the scene. At least some of the transmitted light signal isreflected by an object in the scene and is picked up by the receiver. Achannel 94 carries the light from the transmitter to the object and backto the receiver.

An illuminator 100 in the transmitter (30) applies an amplitudemodulation scheme to the light signal to be emitted, using a chirpmodulation signal 110, wherein the frequency of the chirp modulationsignal changes over time. It is noted that in the present context,amplitude modulation is applied at the electrical domain, e.g., atfrequencies in the range of MHz to GHz. This is not to be confused withcoherent chirp methods in which the wavelength of the light signal(e.g., the color or optical frequency) may change over time.

Illuminator 100 may use various amplitude modulation schemes. Forexample, chirp modulation signal 110 may comprise a sine signal whosefrequency changes over time, an on-off keying (square wave) signal inwhich the width of the pulse changes over time, or a pulsed signal inwhich the width of the pulse is constant, and the spacing between thepulses changes over time. The description that follows refers mainly tochirp LIDAR systems in which the frequency or spacing betweenconstant-width pulses changes over time.

In FIG. 2 , an imager 300 of receiver 40 comprises a photodetector 310followed by a readout circuit 114. For the sake of clarity only FIG. 2depicts only one photodetector and a corresponding readout circuit. Inpractical LIDAR systems, the imager comprises any suitable number ofphotodetectors and corresponding readout circuits. In some embodiments,a single photodetector is used with a Two-Dimensional (2D) scanningpattern, typically a Lissajous pattern. In other embodiments, a vectorphotodetector may be used, with a One-Dimensional (1D) scanning pattern.In yet other embodiments, a 2D array of photodetectors is used.

Readout circuit 114 comprises a processing chain comprising a modulator330, also refers to as a “(de)-modulator”, a Low Pass Filter (LPF) 340,a sampler 344 and an Analog to Digital Converter (ADC) 350.

Photodetector 310 converts light photons into an electrical signal(e.g., electrical current signal) denoted Idiode(t). A quantumefficiency property of the photodetector specifies the number ofcarriers (electrons or holes) generated per photon. A responsivityproperty of the photodetector specifies the ratio between the electricalcurrent output by the photodetector and the total light power fallingupon the photodetector.

In the present example, (de)-modulator 330 comprises an analog mixerthat modulates the electrical signal produced by the photodetector bymultiplying the electrical signal by a chirp shutter signal provided byshutter 320. The chirp shutter signal is represented by a shutterfunction Sh(t). The chirp shutter signal is synchronized to the chirpmodulation signal of the transmitter. The signal output by(de)-modulator 330 is denoted M(t). In some embodiments, the frequencyof the chirp shutter signal is shifted (by a constant frequency shiftvalue) relative to the frequency of the chirp modulation signal.

The signal M(t) contains a low-frequency sine wave signal and ahigh-frequency sine wave signal, wherein both the low-frequency and thehigh-frequency depend on the ToF. LPF 340 is configured to pass thedesired low-frequency sine wave signal and to reject the undesiredhigh-frequency sine wave. In some embodiments, LPF 340 is implemented byaccumulation (integration) of the demodulated signal M(t).

Sampler 344 samples the low-frequency sine wave signal output by the LPFin accordance with a readout cycle scheduled by a readout rate producer348. As will be described below, a single readout cycle contains one ormore chirp sections during which the signal M(t) is accumulated (e.g.,integrated), and the accumulated signal is sampled at the end of thereadout cycle. In the figure, the i^(th) analog sample is denotedV(i·T), wherein ‘T’ denotes the sampling interval and ‘i’ denotes thesample index. ADC 350 converts (e.g., quantizes) the analog samples intodigital samples having a suitable resolution, e.g., 10 or 12 bits persample. The digital sample corresponding to an analog sample V(i·T) isdenoted N(i).

A DSP 400 of receiver 40 comprises a Fast Fourier Transform (FFT) module410, an interpolator 420, and an output formatter—a point cloud creator430 in the present example. FFT 410 receives groups of Nfft digitalsamples from ADC 350, and outputs corresponding groups of Nfft/2 binsdenoted Pk(x). In a practical LIDAR system that comprises multiplephotodetectors, each photodetector requires transforming Nfft digitalsamples into Nfft/2 bins using FFT module 410 (or using multiple FFTmodules 410). Depending on implementation, the FFT bins may bereal-valued or complex-valued. Using only Nfft/2 FFT bins is possiblebecause with a symmetric spectrum half of the FFT bins contain all thespectral information.

A depth frame rate generator 434 schedules depth frames of Nfft readoutcycles, resulting in Nfft/2 FFT bins per photodetector in each readoutcycle. A peak at some FFT bin is indicative of the frequency of thereceived signal and therefore also of the ToF or distance to the object.

The bin resolution of FFT 410 is given by Δf=1/(Nfft−T). It can be shownthat the distance resolution at the FFT output is given by0.5·c/(fmax−fmin), wherein fmin and fmax denote the minimum and maximumfrequencies linearly scanned by the chirp modulation signal.

Interpolator 420 interpolates over the peak FFT bin and one or moreneighbor bins for increasing the accuracy of the depth measurement. Theinterpolator output is denoted Pk(x′). Interpolator 40 may apply anysuitable interpolation method, e.g., linear interpolation. Point cloudcreator 430 produces a point cloud output based on the interpolated FFTbins.

Although in the present example, an FFT module transforms the digitalsamples into a frequency domain, this is not mandatory. In alternativeembodiments other transform methods can be used such as, for example, awavelet transform.

In some embodiments LIDAR system 90 operates in accordance with a timingschedule as described herein. To allow signal accumulation at thereceiver, the full chirp cycle scanning linearly the full frequencyrange, is divided into multiple chirp sections, wherein each chirpsection scans a subrange of the entire frequency range.

The transmitter repeats transmission in accordance with a given chirpsection Q times before starting transmission in accordance with thesubsequent chirp section. At the receiver, the readout cycle forgenerating a single analog sample corresponds to accumulation of thereceived signal over Q repetitions of the same chirp section. At the endof the readout cycle the accumulated signal is sampled once and providedto the ADC for producing a single digital sample. More details on usingchirp sections are described in U.S. patent application Ser. No.16/949,835 cited above.

In some embodiments the transmitter applies decimated chirp techniquesas described in U.S. patent application Ser. No. 16/949,835 cited above.With decimated chirp, a repetition period of transmission is restartedat the beginning of each chirp section. By using decimated chirp, thelaser repetition rate remains below the maximum repetition ratepermitted.

A Lidar Receiver Designed to Mitigate Ambient Light

The accuracy of depth measurements in LIDAR systems are mainly limiteddue to ambient light. Specifically, noise associated with the ambientlight disrupts the interpolation calculation among FFT bins (performedby interpolator 420), which in turn reduces the accuracy of depthmeasurements.

In some embodiments, to mitigate ambient light, the receiver divideseach chirp section into K windows (K being an integer larger than one).Neighboring windows in the same chirp section may partially overlap withone another, in an embodiment. The received signal is accumulatedseparately for the different K windows, resulting in K digital samplesin each readout cycle per photodetector. This is in contrast toconventional un-windowed solution in which a single digital sample isgenerated in each readout cycle). In measuring the depth, a windowcontaining a peak FFT bin is selected, and other windows are discarded.The selected window carries all of the reflected laser signal, but onlyabout 1/K of the noise related to ambient light in the chirp section.The windowing approach therefore result in improving Signal to NoiseRatio (SNR), reducing FFT peak detection failure rate, and increasingthe FFT bin interpolation accuracy.

FIG. 3 is a block diagram that schematically illustrates a receiver 40Amitigating ambient light by dividing chirp sections into multiplewindows, in accordance with an embodiment that is described herein.

Receiver 40A comprises an imager 300A and a DSP 400A. In imager 300A,photodetector 310, shutter 320, (de)-modulator 330 and LPF 340 areessentially the same or similar to corresponding elements in imager 300of FIG. 2 . Imager 300A comprises a readout rate producer 348A, multiple(e.g., K) samplers 344, and multiple (e.g., K) ADCs 350. The readoutrate producer divides each chirp section into K windows denoted W(1) . .. W(k). The signal output by LPF 340 is provided to all samplers 344.Each of the K samplers, however, is enabled for sampling only during itscorresponding window. The samples in each window are accumulated overone or more (e.g., Q) chirp sections of a readout cycle, separately fromother windows, and the K accumulated signals are provided to Krespective ADCs. In accordance with the above scheme, K digital samplesare produced per a readout cycle (per a photodiode).

DSP 400A comprises a depth frame rate generator 434A that controls FFT410 to transform Nfft digital samples of each window into Nfft/2 bins(overall K·Nfft/2 bins per readout cycle). A window selector 440 selectsthe Nfft/2 bins of one of the K windows. For example, window selector440 may select the window for which a certain group of the FFT bins hasa bin with the highest amplitude among the K windows. Since the time (orindex) of each window within the chirp section is directly related todepth, the final FFT is composed by choosing for each chirp section theFFT of the corresponding time window. For example, a window that spans aduration from t_i to t_i+1 (i being the window index), relative to thestart time of the chirp section, contains reflections corresponding todepth ranges from x_i=c·t_i/2 to x_i+1=c·t_i+½, meaning that from thisFFT window only the frequency bins corresponding to that depth range arerelevant. Moreover, in some embodiments, to save power, only the FFTbins related to the time duration of the window are actually computed.

Interpolator 420 interpolates multiple FFT bins of the selected windowfor increasing the accuracy of the depth reading. Point cloud creator430 produces a point cloud output based on the interpolated FFT bins.

FIG. 4 is a block diagram that schematically illustrates a variantimager 300B, in accordance with an embodiment that is described herein.In one aspect, imager 300B differs from imager 300A of FIG. 3 byincluding an amplifier 315 that amplifies the signal output by the photodetector prior to modulation by (de)-modulator 330. In another aspect,imager 300B differs from imager 300A by using an integrating sampler344A instead of a regular sampler 344. The integrating sampler is builtfrom a transconductor stage gm (370) followed by a sampler. Thetransconductor translates input voltage to output current. When theswitch of the sampler is open, the capacitor is reset to a zero voltage(using another switch—not shown in the figure). When the switch isclosed, the capacitor integrates the output current of thetransconductor so that the voltage across the capacitor is given byVcap=Integral (gm·VLPF·dt). In some embodiments, LPF 340 may be omittedbecause the integration by integrating sampler 344A is sufficient.

The inclusion of amplifier 315 and integrators 370 assist in detectinglow-power reflections. It is noted that although in FIG. 4 the imagerincludes both amplifier 315 and integrators 370, this configuration isnot mandatory. In alternative embodiments, the imager may includeamplifier 315 with regular samplers rather than integrating samplers344A, or use integrating samplers 344A without amplifier 315.

FIG. 5 is a diagram that schematically illustrates a chirp section 500divided into multiple windows, in accordance with an embodiment that isdescribed herein.

In the example of FIG. 5 , the laser transmitter is configured totransmit a single laser pulse 504 within each chirp section (e.g., chirpsection 500). Depending on the distance to the object, a reflectionsignal 508 of the transmitted pulse is received after a TOF period 512.Chirp section 500 is divided into K windows denoted W(1) . . . W(K),e.g., by readout rate producer 348A. Any suitable integer number K ofwindows can be used, e.g., K=10 (or even larger) in an embodiment.Neighboring windows may overlap with one another so as not to miss areflection at a window edge. In the present example, a chirp shuttersignal 516 passes reflection signal 508 within the second window W(2).Consequently, window selector 440 selects the FFT output correspondingto window W(2) and all the other windows are discarded. Since in thisscheme the integration time is K times shorter compared to integrationalong the entire chirp section, the amount of shot noise (having aPoisson distribution) associated with ambient light is reduced by afactor that (approximately) equals the square root of K.

As noted above, the signal is integrated for each window across multiplechirp sections that scan a common subrange of the entire chirp, beforebeing sampled.

In some embodiments dividing the chirp sections into windows may becombined with decimated chirp techniques described in U.S. patentapplication Ser. No. 16/949,835 cited above. Next are described examplevariants of imager 300A.

Reducing Variations in Power Consumption Caused by the Chirp ShutterSignal

The chirp shutter signal produced by shutter 320 is typicallydistributed to all (de)-modulators 330 of imager 300. Since powerconsumption depends on the chirp frequency, the power (or electricalcurrent) consumption becomes time dependent because of the chirp shuttersignal. As a result, the Internal Resistance (IR) drop is also timedependent, which negatively affects the image uniformity of the LIDARsystem.

In some embodiments, to compensate for the effect of the ascending chirpshutter signal, imager 300 produces a descending chirp shutter signalthat is synchronized to the ascending chirp shutter signal. Moreover,the descending chirp shutter signal is applied to a load similar to theload circuit to which the ascending chirp shutter signal is applied. Itis noted that the ascending and descending chirp shutter signals areapplied synchronously to one another in imager 300 of the receiver andin illuminator 100 of the transmitter.

In one embodiment, the descending chirp shutter signal is applied to aload comprising a dummy shutter circuit. In this embodiment, the overallpower consumption may increase due to the power consumed by theadditional dummy shutter circuit. In another embodiment, one subset of(de)-modulators 330 in imager 300 is applied the ascending chirp shuttersignal, while another different subset of (de)-modulators 330 is appliedthe descending chirp shutter signal.

Although in the present example, the original chirp shutter signal is anascending chirp signal and the additional chirp shutter signal is adescending chirp signal, this is not mandatory. In alternativeembodiments, the original and added chirp shutter signals may bedescending and ascending chirp shutter signals, respectively.

FIG. 6 is a diagram that schematically illustrates chirp signals andrelated average power consumption, in accordance with an embodiment thatis described herein. FIG. 6 depicts an ascending chirp signal 550 whosefrequency increases linearly with time, in which case the resultingaverage power consumption 554 also increases linearly with time. FIG. 6further depicts a descending chirp signal 558 whose frequency decreaseslinearly with time, resulting in average power consumption 562 that alsodecreases linearly with time. The total average power consumption 566 isthe instantaneous sum of the separate average power consumptions 554 and562, which is constant (or in practice close to constant) over time.

Imager (De)-Modulator Implemented Using a Switched Circuit

In the embodiments described above, (de)-modulator 330 was implementedusing an analog mixer that multiplies the signal produced by thephotodetector by the chirp shutter signal Sh(t). Next are describedalternative implementations of (de)-modulator 330 using various types ofswitched circuits. The switched circuits may be advantageous compared tousing the mixer for various reasons such as: (i) their implementationsimplicity, (ii) they typically consume less power than the mixer(because no DC biasing current is involved), and (iii) they haveimproved accuracy because the shutter signal is seen in terms of timing(edge position).

FIGS. 7A and 7B are block diagrams that schematically illustrate imagers(300C) in which the (de)-modulator is implemented using a Track-and-Hold(TAH) circuit. In FIGS. 7A and 7B a TAH circuit 372 receives the analogsignal from the photodetector (or from amplifier 315) as input. Underthe control of the chirp shutter signal Sh(t), the output of the TAHcircuits tracks the input signal or holds the last value tracked of theinput signal.

TAH circuit 372 can be implemented in various ways. For example, in FIG.7B, the TAH circuit is implemented using a switch S and a capacitor C.In this implementation the energy of any pulse that arrives during thetime the shutter signal is low is lost and not integrated (half of thetime for a 50% duty cycle shutter function).

FIGS. 8A and 8B are block diagrams that schematically illustrate imagers(300D) in which the (de)-modulator is implemented using aSample-and-Hold (SAH) circuit. In FIGS. 8A and 8B a SAH circuit 376receives the analog signal from the photodetector (or from amplifier315) as input. Under the control of two chirp shutter signals Sh_p(t)and Sh_n(t), the output of the TAH circuit samples the input signal andholds the last value sampled. The signals Sh_p(t) and Sh_n(t) are inopposite phases, meaning that when Sh_p(t) is high, Sh_n(t) is low, andvice versa.

SAH circuit 376 can be implemented in various ways. For example, in FIG.8B, the SAH circuit is implemented using two switches S and twocapacitors C. The switches operate in opposite phases so that when oneswitch is closed the other switch is open. In some embodiments, theshutter produces a single control signal Sh(t), e.g., serving asSh_p(t), and the other control signal, e.g., Sh_n(t) is derived fromSh(t) using a logical inverter (not shown).

In some embodiments, to exploit the full signal energy, the imageremploys differential TAH and SAH circuits, as described herein. Usingthe differential TAH and SAH circuits also reduces or eliminates flickernoise, also referred to as “1/f noise”, which is a low-frequency noisefor which the noise power is inversely proportional to the frequency.Reducing flicker noise may be achieved using chopping, which is atechnique for reducing amplifier offset voltage, but since flicker noiseis a low-frequency noise, it is effectively reduced by the choppingtechnique. Similarly, with an offset voltage method, the DC signaltogether with the low frequency flicker noise are modulated by thechopping, and thus transferred to a (high) frequency that is outside thebandwidth of the given amplifier.

FIGS. 9A-9C are a block diagram and electronic circuit diagrams thatschematically illustrate an imager 300E employing a differential TAHcircuit 372A, and example circuits implementing the differential TAH, inaccordance with embodiments that are described herein.

In FIG. 9A, a shutter 320A produces two chirp shutter signals denotedSh_p(t) and Sh_n(t), The two chirp shutter signals operate in oppositephases, meaning that when Sh_p(t) is high, Sh_n(t) is low, and viceversa. Differential TAH circuit 374A has a differential outputcomprising two track-and-hold outputs denoted M_p(t) and M_n(t). TheM_p(t) output is tracked when Sh_p(t) is high and sampled when Sh_p(t)is low. The M_n(t) output is tracked when Sh_n(t) is high and sampledwhen Sh_n(t) is low.

In FIGS. 9B and 9C, differential TAH circuit 374A comprises two TAHcircuits, each of which comprising a switch S and a capacitor C. In FIG.9C, shutter 320 produces a single chirp shutter signal Sh(t) thatcontrols the first TAH circuit, and a logically inverted version ofSh(t) controls the second TAH circuit.

FIGS. 10A-10C are a block diagram and electronic circuit diagrams thatschematically illustrate an imager 300F employing a differential SAHcircuit 376A, and example circuits implementing the differential SAHcircuit, in accordance with embodiments that are described herein.

In FIG. 10A, a shutter 320A produces two chirp shutter signals Sh_p(t)and Sh_n(t), operating in opposite phases. Differential SAH circuit 376has a differential output comprising two sample-and-hold outputs denotedM_p(t) and M_n(t). The M_p(t) output samples the input signal on thefalling edge of Sh_p(t) and holds it for a full period. The M_n(t)output samples the input signal on the falling edge of Sh_n(t) and holdsit for a full period.

In FIGS. 10B and 10C, differential SAH circuit 376A comprises two SAHcircuits, each of which comprising two switches S and two capacitors C.In FIG. 10B, Sh_p(t) controls the first switch in the first SAH circuitand the second switch in the second SAH circuit, and Sh_n(t) controlsthe second switch in the first SAH circuit and the first switch in thesecond SAH circuit. In FIG. 10C, shutter 320 produces a single chirpshutter signal Sh(t) that controls the first switch in the first SAHcircuit and the second switch in the second SAH circuit. A logicallyinverted version of Sh(t) controls the second switch in the first SAHcircuit and the first switch in the second SAH circuit.

The embodiments described above are given by way of example, and othersuitable embodiments can also be used. For example, although in theembodiments described above the shutter (e.g., 320) that produces thechirp shutter signal is separate from the corresponding switched circuit(e.g., 372 or 376), in other embodiments the shutter and the switchedcircuit are combined together as a shutter.

The configurations of LIDAR system 10 of FIG. 1 , LIDAR data flows ofFIGS. 2 and 3 , and imager variant implementations of FIGS. 7A, 7B, 8A,8B, 9A-9C, and 10A-10C, are given by way of example, and other LIDARsystem, LIDAR data flows, and imager variant implementations can also beused. Elements that are not necessary for understanding the principlesof the present invention, such as various interfaces, addressingcircuits, timing and sequencing circuits and debugging circuits, havebeen omitted from the figures for clarity.

Some elements of transmitter 30 and receiver 40 of LIDAR system 10, suchas imager 42, signal processor 20, and DSPs 400 and 400A may beimplemented in hardware, e.g., in one or more Application-SpecificIntegrated Circuits (ASICs) or FPGAs. Additionally or alternatively,signal processor 20 and DSPs 400 and 400A can be implemented usingsoftware, or using a combination of hardware and software elements.

In some embodiments, some functions of, signal processor 20 and DSP 400and/or DSP 400A, may be carried out by general-purpose processors, whichare programmed in software to carry out the functions described herein.The software may be downloaded to the processors in electronic form,over a network, for example, or it may, alternatively or additionally,be provided and/or stored on non-transitory tangible media, such asmagnetic, optical, or electronic memory.

Although the embodiments described herein mainly address LIDAR systems,the methods and systems described herein can also be used in otherapplications, such as in distance measurement applications, and inreflectometry.

It will be appreciated that the embodiments described above are cited byway of example, and that the following claims are not limited to whathas been particularly shown and described hereinabove. Rather, the scopeincludes both combinations and sub-combinations of the various featuresdescribed hereinabove, as well as variations and modifications thereofwhich would occur to persons skilled in the art upon reading theforegoing description and which are not disclosed in the prior art.Documents incorporated by reference in the present patent applicationare to be considered an integral part of the application except that tothe extent any terms are defined in these incorporated documents in amanner that conflicts with the definitions made explicitly or implicitlyin the present specification, only the definitions in the presentspecification should be considered.

1. A LIDAR system, comprising: a transmitter, which is configured todirect a sequence of illumination pulses toward a scene; a receiver,comprising: an array of photodetectors, which are configured to receiveoptical radiation reflected from the scene and to output respectivesignals in response to the received optical radiation; a shutter, whichis configured to modulate the signals output by the photodetectors byapplying a chirp shutter function, having a selected chirp period, tothe signals; and a readout circuit, which is configured to sample anddigitize the modulated signals in each of a plurality of samplingwindows, which span the chirp period, thereby generating a correspondingplurality of digitized output signals; and a processor, which isconfigured to select respective sampling windows for the photodetectors,and to process the digitized output signals in the selected respectivesampling windows to generate a depth map of the scene.
 2. The LIDARsystem according to claim 1, wherein the readout circuit is configuredto integrate the sampled signals over each of the sampling windows. 3.The LIDAR system according to claim 1, wherein the processor isconfigured to calculate a time-frequency transform for the samplingwindows in a given chirp period, and to select a sampling window in thegiven chirp period based on partial subsets of the frequency-domainbins, corresponding respectively to the sampling windows.
 4. The LIDARsystem according to claim 3, wherein the processor is configured tocalculate the time-frequency transform by calculating only the partialsubsets of the frequency-domain bins.
 5. A LIDAR system, comprising: atransmitter, which is configured to direct a sequence of illuminationpulses toward a scene; a receiver, comprising: an array ofphotodetectors, which are configured to receive optical radiationreflected from the scene and to output respective signals in response tothe received optical radiation; a shutter, which is configured tomodulate the signals by applying an ascending chirp shutter function,having a selected chirp period, to the signals output by at least afirst group of the photodetectors and simultaneously applying adescending chirp shutter function, having the same chirp period as theascending chirp shutter function, to a load; and a readout circuit,which is configured to sample and digitize the modulated signals togenerate digitized output signals; and a processor, which is configuredto process the digitized output signals to generate a depth map of thescene.
 6. The system according to claim 5, wherein the load comprises asecond group of the photodetectors, different from the first group ofthe photodetectors.
 7. A LIDAR system, comprising: a transmitter, whichis configured to direct a sequence of illumination pulses toward ascene; a receiver, comprising: an array of photodetectors, which areconfigured to receive optical radiation reflected from the scene and tooutput respective signals in response to the received optical radiation;a shutter, which is configured to modulate the signals output by thephotodetectors by applying a chirp shutter function, having a selectedchirp period, to the signals, wherein the shutter comprises a switchedcircuit comprising a differential track-and-hold or a differentialsample-and-hold circuit, which is controlled to apply the chirp shutterfunction; and a readout circuit, which is configured to sample anddigitize the modulated signals to generate digitized output signals; anda processor, which is configured to process the digitized output signalsto generate a depth map of the scene.
 8. The LIDAR system according toclaim 7, wherein the switched circuit is configured to reduce flickernoise using a chopping technique.
 9. The LIDAR system according to claim7, wherein the switched circuit comprises an integrating samplercomprising a transconductor stage followed by a sampler.